Many lighting devices are required to project their output light into beam patterns which are elongated in a defined plane commonly the horizontal plane--and shortened in a plane normal to the defined plane. A typical requirement would require a total beam spread of 10 degrees in the horizontal and 5 degrees in the vertical plane. In order to comply with these requirements optical systems must collect the light energy created by the device and direct it into the required beam pattern. It is usually impossible to redirect all the created light into the projected beam and some will be misdirected or lost. Improved designs reduce the percentage of created light that is lost.
Light emitting diode (LED) lamps for signaling application are being used in a number of lighting devices. The most common LED lamp is one with a cylindrical body and lens top. The lens collects and concentrates the light emitted by the light emitting diode element to form an intense output beam of light. The symmetry of the optics concentrate light both in the horizontal and vertical planes to an equal degree. Since many applications require elongated output light beams it is commonplace to group these discrete LED lamps in an elongated pattern within a lighting device to project the desired beam pattern. This design has a serious drawback in that the lens top LED lamp is very limited in its capacity to collect the light emitted by the diode element within its body. Generally, LED lamps that incorporate integral lenses to concentrate their light energy lose the ability to collect that light energy as the degree of concentration increases. Thus, although the lens top LED lamp produces concentrated output, it unfortunately fails to employ much of the energy being created.
Prior art design U.S. Pat. No. 4,654,629 issued to Bezos directs the output light beam from the lens top LED lamp into a secondary lens system. Although this is a workable design, it is deficient because most of the photometric energy has been lost before it can be captured by the secondary lens system. This lost energy is the result of unwanted refraction and internal reflection at the surface of the LED lamp. Additional prior art can be found in U.S. Pat. No. 4,009,394 issued to Mierzwinski. This device is a cylindrical lens for transmitting infrared light. It is an attempt to employ a greater percentage of the created light energy in the projected beam of an infrared transmitting device. It is constructed with a straight cylindrical lens surrounded by four reflective walls. Light from the infrared LED source which would otherwise be misdirected or lost is reflected from the reflective walls and contributes to the usable output energy. In order to reduce losses between the cylindrical lens and LED lamp "cap" which would occur "with air between different optical surfaces", the design bonds the "cap" to the lens. Although apparently successful for his purpose, the Mierzwinski patent would have little value for many lighting devices because the light collected is not controlled sufficiently to be directed into a projected beam pattern meeting a specification with limited vertical and/or horizontal beam spread. Cylindrical surfaces can effectively redirect light into a projected beam with a limited horizontal and/or vertical beam spread. However, meeting this objective requires both a definite and consistent relationship between the apparent or virtual point of emission of the light rays entering the lens and the foci of the lens surface. The Mierzwinski design discloses no consistency of relationship between the foci of the surface lens and the apparent point of emission of the light rays which enter it. Light reflecting from the walls of the Mierzwinski patent enter the cylindrical lens surface at a variety of angles and thus appear to originate from a multiplicity of sources from a plurality of locations each with a different geometrical relationship to the focal line. Furthermore, light created by the LED lamp which passes directly into the cylindrical lens surface will not be accurately redirected because the cylindrical surface has a focus line and the angular relationship and distance between the light source and focus line varies as one proceeds in the horizontal plane along the face of the lens. Therefore, the Mierzwinski design does not accurately control the relationship between the apparent or in his case the actual light source and the lens foci for light heading directly into the lens or for light entering the lens after reflection from the side walls. For this reason the design cannot efficiently produce the limited defined projected beam necessary for a signal device. Finally, light rays both reflected and direct which enter the cylindrical surface from other then along the axis of device experience different degrees of refraction due to the different lens contour they encounter. FIG. 4 of the patent is the sectional view of a vertical plane through the central axis and shows a curved lens surface 13. The shape of lens surface 13 would change if the vertical plane were angled from the central axis resulting in a change in the optical effect of that lens. Thus the Mierzwinski design would not achieve the control of the created light necessary for most signal devices because the created light rays experience a variety of lens contours depending upon their angle of divergence from the axis of the device.